In a physical vapor deposition (PVD) chamber, a region exists between a plasma and its surrounding surfaces, such as powered, grounded, or floating electrodes, referred to as the dark space region or sheath region. A typical PVD chamber may use a radio frequency (RF) power source to form the plasma. As the frequency of the RF source increases, the plasma density increases and the sheath width decreases. The inventors have observed that this phenomenon can negatively affect the desired plasma geometry at the plasma boundary and cause secondary plasma ignition in unwanted areas. In certain PVD chambers, a grounded electrode (referred to herein as the grounded shield) acts as the main current return path from the plasma back to the generator. In the case of RF driven plasma discharges, the plasma potential can be in the region of a few tens to a few hundred volts positive with respect to the grounded shield. This potential difference coupled with the high plasma-ion density can cause undesirable sputtering of the grounded shield, which can be a source of contamination of the chamber and/or the substrate.
Accordingly, the inventors have provided methods and apparatus for reducing the sputtering of a grounded shield in a process chamber.